The Minecraft security system Mojang should have created to stop griefers.

1. Install the plugin.
2. The whitelist is enabled by default.
3. The first person to join the server is given operator. This person adds players to the whitelist.
4. All players who join subsequently are put into spectator mode. They can't do anything to your server.
5. When you whitelist a player, they are moved from spectator mode to the default game mode of your server.